I do believe that vendor-interoperable SIP exists today, but I also know that I can't call >99.9..9% of the people in the world via SIP, and 100% of the ones I

I do believe that vendor-interoperable SIP exists today, but I also know that I can't call >99.9..9% of the people in the world via SIP, and 100% of the ones I practically do call. This is a very different story from e.g. email – even though most people I communicate with use either Gmail or some hosted Outlook service, I don't have to on my side (with some limitations around deliverability etc. lately).